As shown in FIG. 2, a propeller type wind turbine used in a wind turbine generator includes a plurality of wind turbine blades (usually three wind turbine blades, and a structure having three wind turbine blades will be described herein). A blade pitch angle is controlled so that a predetermined rotation speed and output are obtained in accordance with wind conditions. According to wind turbines of recent years, as a length of the wind turbine blade is increased, individual pitch angle control is included in blade pitch angle control (e.g., Patent Citation 1). According to this control, an individual pitch angle demand that is independent between the wind turbine blades calculated in accordance with an azimuth angle or a load of each wind turbine blade is added to a collective pitch angle demand that is the same for all of the wind turbine blades, and the blade pitch angle is controlled based on the added value. With this individual pitch angle control, it is possible to reduce an amount of variation in rotor out-of-plane load (load that is perpendicular to a rotation plane of the wind turbine rotor), and to increase a fatigue life of the wind turbine.
When an alarm is detected, shutdown control is performed to immediately close the blade pitch angle of the wind turbine blades and to swiftly shut down the wind turbine. When the shutdown control is performed in a wind turbine having the individual pitch angle control, the blade pitch angles of the wind turbine blades are different from each other also when the shutdown is started, and control for closing the blade pitch angles of all of the wind turbine blade is performed from this state at the same pitch rate (that is, feathering is performed to bring the blade pitch angle in parallel to a wind direction). In this case, during the shutdown control, the wind turbine rotor rotates in a state where the blade pitch angles of the wind turbine blades are different from each other irrespective of the azimuth angle or the load, and an aerodynamic imbalance is generated in the wind turbine rotor. An excessive load generated in the entire wind turbine by the aerodynamic imbalance is one of design constraints of the wind turbine, and it is desired to reduce the aerodynamic imbalance.
In order to solve such a problem, Patent Citation 2 proposes the following control technique. For a wind turbine blade (first blade) having a blade pitch angle closest to a fine side, the blade pitch angle is closed at a prescribed maximum pitch rate. For another wind turbine blade having a blade pitch angle closer to a feather side, the blade pitch angle is closed at a pitch rate smaller than the maximum pitch rate until the blade pitch angle of the first blade catches up with the latter wind turbine blade. After the blade pitch angle becomes the same as the blade pitch angle of the first blade, the blade pitch angle is closed at the same pitch rate as that of the first blade, and the blade pitch angles of the three wind turbine blades are allowed to swiftly match with each other for feathering (shut down the wind turbine).    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-201952    Patent Document 2: U.S. Patent Application Publication No. 2009/148286    Patent Document 3: PCT International Publication No. WO2009/139066